1. Field of the Invention
This invention relates to the chemical arts. In particular, it relates to a process for degassing liquid sulfur.
2. Discussion of the Related Art
An important process for removing hazardous hydrogen sulfide (H.sub.2 S) from various waste gases, including gases produced during the refining of petroleum products, is known as the Claus process. It involves the following net reaction: EQU H.sub.2 S+1/2O.sub.2 .fwdarw.H.sub.2 O+S (1)
Unfortunately, the handling of the liquid sulfur produced by plants using the Claus process can be extremely difficult. Many fires and explosions have occurred during the storage and transport of liquid sulfur, because of the accumulation of toxic and highly flammable H.sub.2 S gas.
The H.sub.2 S gas is produced by the gradual decomposition of polysulfides present in the liquid sulfur as represented by the following equation: ##EQU1## The dissolved H.sub.2 S then passes into the gas phase by physical desorption. ##EQU2## Consequently, to be successful, a degassification process must not only remove H.sub.2 S, but must also remove polysulfides, to prevent the subsequent generation of additional H.sub.2 S.
It is a disadvantage of modern, commercial degassification processes that they require large, complex and, accordingly, expensive equipment. For example, in one process, known as the Shell process, degassing takes place in a storage tank or sulfur pit equipped with stripping columns, where liquid sulfur is vigorously agitated by bubbling air therethrough at atmospheric pressure. The stripping columns are open at their tops and bottoms to allow the sulfur to circulate at a rate of a few hundred times per hour. The bubble air, together with an additional flow of air, is then used as a low pressure sweep gas to displace the gases produced by the degassification process. The low pressure gases so produced are then fed to an incinerator where the H.sub.2 S is oxidized to SO.sub.2 and released to the atmosphere. Depending on the design, a liquid or gaseous catalyst, such as ammonia, ammonium thiosulfate, urea, morpholine, or an alkanol amine may be added for accelerating the decomposition of the polysulfide into H.sub.2 S.
In an alternative process, known as the SNEA process, degassing takes place by repeated circulation and spraying of the liquid sulfur into the sulfur pit. Release of dissolved H.sub.2 S is achieved by spraying liquid sulfur through jets at a specific velocity. Ammonia, injected at the suction of the recirculation pump, is typically used as a catalyst. After the H.sub.2 S gas is released, it is removed by a sweep gas and fed to an incinerator. Both the stripping columns used in the Shell process and the circulation/spraying equipment used in the SNEA process are costly and require a large amount of space. Further, it is a disadvantage of both processes that they require the additional step of having to incinerate the H.sub.2 S-containing sweep gases.
It is another disadvantage of modern, commercial degassification processes that they require a relatively long retention time in the sulfur pit. For example, the Shell process typically requires the liquid sulfur to be recirculated through the stripping columns for about twenty to about twenty-four hours, while the SNEA process typically requires the liquid sulfur to be recirculated through the spray jets for about twenty-four to about thirty hours.
Accordingly, there has existed a definite need for a degassification process that not only effectively reduces the H.sub.2 S content of liquid sulfur, but is simple, requires a minimum amount of space, and is inexpensive. There has existed a further need for a process that takes a relatively short amount of retention time to achieve the desired liquid sulfur degassification. There has existed a still further need for a process that does not require incineration of the H.sub.2 S gas released from the liquid sulfur. The present invention satisfies these and other needs, and provides further related advantages.